Anatomy, Thorax, Wall

Introduction

The thoracic wall consists of a bony framework that is held together by twelve thoracic vertebrae posteriorly which give rise to ribs that encircle the lateral and anterior thoracic cavity. The first nine ribs curve around the lateral thoracic wall and connect to the manubrium and sternum. Ribs 10-12 are relatively short and attach to the costal margins of the ribs just above them. Ribs 10-12, due to their short course, they do not reach the sternum.

The first seven ribs are termed true ribs and attach to the manubrium and directly attach to the body of the sternum. Ribs eight to ten only attach to the inferior part of sternum via the costal cartilages. Ribs 11-12 are termed floating ribs because they do not attach directly to the sternum. Ribs eight to ten are known as false ribs because they lack direct attachment to the sternum. At the level of the spine, the ribs articulate with the costal facet of two opposing vertebrae. An articular capsule surrounds the head of each rib, and the attachment to the transverse process is made with the help of the radiate ligament. Once the ribs leave the vertebrae, they gently curve around the lateral thoracic wall and approach the anterior wall of the thoracic cavity.[1]

The vertical bone of the chest, the sternum, defines the anterior chest wall. The three separate bone segments of different size and shape that make up the sternum include 1) the thick manubrium, 2) long body of the sternum, and 3) the xiphoid process. It develops independently of the ribs. In sporadic cases, the sternum may not fully form, and the underlying heart may be exposed.

The most superior portion of the sternum is the manubrium, and it is also the first to form during embryogenesis. The sternal body and xiphoid process soon follow the manubrium in development. Anatomically, the manubrium is located at the level of thoracic vertebral bodies T3 and T4. The manubrium is also the widest and thickest segment of the sternum. During a physical exam of the chest, one noticeable feature of the manubrium is the presence of the suprasternal notch. On either side of this notch, one will feel the thick attachment from the clavicles. For access to the superior mediastinum, suprasternal goiter or thymus, some thoracic surgeons will only make a midline incision in the manubrium.

The sternal body is located at the level of vertebral bodies T5-T9. It covers a significant portion of the mid-chest and is very strong. To access the chest cavity, surgeons usually cut through the sternum with a mechanical saw.

The xiphoid process is a thin and very small bone. Its size may vary from two to five cm, and its shape is also variable. The xiphoid may appear bifid, oval or be curved inwards/outwards. In younger individuals, the xiphoid is mostly cartilaginous but is nearly wholly ossified by age 40. By the age of 60 and over, the xiphoid is almost certainly completely calcified. To perform pericardiocentesis safely the needle has to be placed directly underneath the xiphoid because the heart is just a few fingerbreadths below.[1][2]

Structure and Function

The thoracic cavity subdivides into three compartments; the mediastinum and two pleural cavities, one on each side. The mediastinum is the median compartment containing the heart and great vessels; whereas, the pleural cavities contain the lungs. The thoracic cage protects the lungs and the heart as well as provide attachments for the muscles of the thorax, upper extremities, back, and the abdomen. It communicates superiorly with the neck via the thoracic outlet and inferiorly separates the abdomen by the respiratory diaphragm.[1]

The boundaries of the thoracic wall are important landmarks used by clinician and surgeons for various procedures including sternotomy, pericardiocentesis in patients with cardiac tamponade and thoracentesis for pleural effusion. The thoracic wall is bounded anteriorly by the sternum and costal cartilages; laterally by the ribs and intercostal spaces; posteriorly by the thoracic vertebrae and intervertebral discs; superiorly by the suprapleural membrane and inferiorly by the respiratory diaphragm.[1][2]

Embryology

The formation of somite begins as the paraxial mesoderm starts to spiral into an organized cell called somitomere. These somitomeres cluster together by epithelium and separate from the presomitic paraxial mesoderm to form individual somites. The differentiation in somite forms the cartilage of the vertebrae, ribs, the muscle of the rib cage, limb and even dermis of the skin.

Blood Supply and Lymphatics

Three arteries supply each intercostal space; the posterior intercostal artery and two branches of anterior intercostal arteries. These intercostal blood vessels run along with the nerves between the internal intercostal muscle and innermost intercostal muscles in the costal groove. They are arranged in order from superior to inferior: vein, artery, and nerve.

The posterior intercostal artery for first two intercostal spaces is fed from the superior (supreme) intercostal artery. This artery arises from the costocervical trunk of the subclavian artery. The remaining pair of posterior intercostal arteries from 3rd - 11th intercostal spaces and a pair of subcostal arteries emerge directly descending thoracic aorta.[1]

The anterior intercostal arteries of 1st–6th intercostal spaces are branches of the internal thoracic artery which derive from the first portion of the subclavian artery. The anterior intercostal arteries of 7th–9th intercostal spaces are branches of the musculophrenic artery which is a terminal tributary of the internal thoracic artery. The anterior and posterior intercostal arteries anastomose laterally in the costal groove.[3]

The corresponding posterior intercostal vein drains into azygos or hemiazygos veins and the corresponding anterior intercostal veins drain into internal thoracic or musculophrenic veins. The lymphatics of the thoracic wall drains into parasternal lymph nodes and intercostal lymph nodes. The parasternal lymph nodes and intercostal lymph nodes from the upper thorax drain into the bronchomediastinal trunk, whereas, the intercostal nodes from the lower thorax drains into the thoracic duct.[4]

Nerves

The thoracic wall is primarily innervated by the intercostal nerves, which are the anterior rami of spinal nerves of T1-T11 and the anterior ramus of T12 is a subcostal nerve. Each intercostal nerve supplies a dermatome and a myotome. Only the anterior ramus of T1 forms the lower trunk of the brachial plexus; the remaining intercostals do not form a plexus.[1][2]

Muscles

There are three intercostal muscles; externally intercostal, internal intercostal and innermost intercostal muscles. These muscles are present in the intercostal spaces and the intercostal nerves and blood vessels run between them. The most superficial layer is the external intercostal muscle. The external intercostal muscles extend posteriorly from the rib tubercle to the costochondral junction anteriorly where the anterior (external) intercostal membrane takes the place of the muscle fibers.[2]

The internal intercostal muscle forms the intermediate layer. This muscle extends anteriorly from the sternum to the rib cage posteriorly where the muscle fibers are replaced the posterior (internal) intercostal membrane. The innermost intercostal muscle forms the deepest layer and is lined internally by the endothoracic fascia, which in turn is lined internally by the parietal pleura.[2][5]

Physiologic Variants

The difference in size of the sternum in both genders may provide essential clues in determining the skeletal remains during forensic evaluation.[1]

Surgical Considerations

Understanding the anatomy of the thorax is vital, as it provides access to the heart, great vessels, lungs, diaphragm, and mediastinum.

The intercostal nerve block is a procedure through which local anesthetic agent is injected around the intercostal nerve between the paravertebral line and the area of required anesthesia for different surgical procedures. However, the surgeon needs to anesthetize the adjacent nerve as well because of considerable overlapping of contiguous dermatomes occurs.[6]

Median sternotomies are the most commonly performed osteotomy in the world and a standard incision for thoracic and cardiac surgery. The median sternotomy is a critical procedure in which the surgeon splits the sternum in the median plane to gain access to the heart, great vessels, as well as the lungs.[7]

Flail chest is an extremely painful injury affecting the respiration, often caused by blunt chest trauma, resulting in multiple successive rib fractures. The fractured ribs of the anterior and/or lateral chest wall move inward on inspiration and outward on expiration (paradoxical) and may be associated with pneumothorax and/or contusion of the heart and lung. Today, flail chest management is through pain control, pulmonary toilet, and early ambulation. It is not common to surgically repair the ribs.[8]

Clinical Significance

The vitality of the organs, vessels, and nerves located within the thoracic cavity predispose it to be a location of high clinical significance.

The sternum is a commonly used site for bone marrow aspiration because it possesses hematopoietic marrow throughout life. However, the surgeon needs to exercise great care because if the sternal puncture is improperly executed, the needle can pierce the structures related to the posterior surface of the manubrium such as the left brachiocephalic vein in the upper part and aortic arch in the lower part.[9]

Thoracentesis is a diagnostic procedure that is performed by the surgeon to remove excess fluid from the thoracic cavity for both diagnostic or therapeutic purposes. This procedure can be performed in two positions depending on the comfort of the patient. If the patient is lying supine, the needle placement is typically in the midaxillary line between the 6th- 8th ribs, and if the patient is in an upright or seated position, then the needle is placed between the 9th- 10th rib posteriorly in the midscapular line.[10] However, in both cases, a too low needle insertion can result in puncture of the liver or the spleen

Pericardiocentesis is also a necessary procedure as it can be life-saving in a patient with cardiac tamponade. Pericardiocentesis may be guided using surface landmarks (“blindly”) in emergencies or electrocardiogram/echocardiography may be used to advance the needle to avoid complications. The needle is inserted 1-2cm lateral to the apex beat within the fifth, sixth or seventh intercostal space to remove excess fluid from the pericardial sac. The needle is advanced over the superior border of the rib to avoid intercostal nerves and vessels. If this performed improperly, the needle can puncture left ventricle or causes pneumothorax.[11]

In patients with the thoracic outlet syndrome (TOS), there is compression of the neurovascular structures in the thoracic outlet, resulting in pain, numbness, weakness, muscle wasting, fatigue in the upper limbs as well as ischemia. The brachial plexuses of nerves (C5, C6, C7, C8, and T1) and subclavian vessels (artery and vein) are closely related to the first rib as well as the clavicle as they enter the upper limb. These structures may undergo compression between the scalene muscles and the first rib. Pancoast tumors, as well as the presence of a rudimentary cervical rib, are among the potential causes of TOS.[12]

Other Issues

The chest wall deformities, including pectus excavatum and pectus carinatum, are one of the most common congenital chest wall defects seen in young people. Surgical correction is needed in some people to avoid complications which may lead to heart and lungs dysfunction. However, these techniques require aggressive resection of the cartilage and rib cage, leading to severe post-operational complications such as infections, and delayed healing.[13]